On Fri, Jul 01, 2022 at 08:40:02PM +0200, Helge Deller wrote: > In the kernel image vmlinux.lds.S linker scripts the .altinstructions > and __bug_table sections are 32- or 64-bit aligned because they hold 32- > and/or 64-bit values. > > But for modules the module.lds.S linker script doesn't define a default > alignment yet, so the linker chooses the default byte alignment, which > then leads to unnecessary unaligned memory accesses at runtime. > > Usually such unaligned accesses are unnoticed, because either the > hardware (as on x86 CPUs) or in-kernel exception handlers (e.g. on hppa > or sparc) emulate and fix them up at runtime. > > On hppa the 32-bit unalignment exception handler was temporarily broken > due another bad commit, and as such wrong values were returned on > unaligned accesses to the altinstructions table. OK so some bad commit broke something which caused bad alignment access on altinstructions... But why on modules?! I am not aware of modules using alternatives, given that alternatives are hacks to help with bootup. For modules we can use other things like jump labels, static keys. So I don't understand still how this happened yet. > This then led to > undefined behaviour because wrong kernel addresses were patched and we > suddenly faced lots of unrelated bugs, as can be seen in this mail > thread: > https://lore.kernel.org/all/07d91863-dacc-a503-aa2b-05c3b92a1e39@xxxxxxxx/T/#mab602dfa32be5e229d5e192ab012af196d04d75d > > This patch adds the missing natural alignment for kernel modules to > avoid unnecessary (hard- or software-based) fixups. Is it correct to infer that issue you found through a bad commit was then through code inspection after the bad commit made the kernel do something stupid with unaligned access to some module altinstructions section ? Ie, that should not have happened. I'd like to determine if this is a stable fix, a regression, etc. And this is not yet clear. Luis > > Signed-off-by: Helge Deller <deller@xxxxxx> > --- > scripts/module.lds.S | 2 ++ > 1 file changed, 2 insertions(+) > > -- > v2: updated commit message > > diff --git a/scripts/module.lds.S b/scripts/module.lds.S > index 1d0e1e4dc3d2..3a3aa2354ed8 100644 > --- a/scripts/module.lds.S > +++ b/scripts/module.lds.S > @@ -27,6 +27,8 @@ SECTIONS { > .ctors 0 : ALIGN(8) { *(SORT(.ctors.*)) *(.ctors) } > .init_array 0 : ALIGN(8) { *(SORT(.init_array.*)) *(.init_array) } > > + .altinstructions 0 : ALIGN(8) { KEEP(*(.altinstructions)) } > + __bug_table 0 : ALIGN(8) { KEEP(*(__bug_table)) } > __jump_table 0 : ALIGN(8) { KEEP(*(__jump_table)) } > > __patchable_function_entries : { *(__patchable_function_entries) }
